There are electrical systems made up of plural components, some of which are inherently unlikely to fail, or impractical of replication, while others are of nature more susceptible of failure or of replication. Redundancy is an engineering principal by means of which the likelihood of system failure is reduced by replication of parts so that if any components fail, a duplicate may be substituted automatically or with slight delay.
For an example of a system where partial redundancy is desirable, consider the apparatus used to indicate the quantity or depth of fuel in the tanks of a large aircraft. The sensors or depth probes in the fuel tanks are physical structures not apt to any kind of failure, and the indicating devices observed by the pilot are likewise sturdy mechanical structures from which continued satisfactory performance may be expected. However, other components in such a system, which may be quite complex for a large aircraft and may include amplifiers, power supplies, multiplexers, microprocessors, and so on, are electronic in nature and statistical failure within a given period of time can be predicted with some certainty.
Difficulty has been experienced in the past, however, in providing electronic components which would not debase each other's accuracy when used concurrently, yet would be able to replace one another in case of failure. An example of this is operational amplifiers, which when connected in parallel to a current signal source divide the signal between themselves, so that neither gives an output correctly representative of the signal.